Activity of a component

The experimental determination of the relationship between the electronic concentration and the partial pressure or activity of a component is commonly the best method to determine the type of disorder in a material. 

The two cases relating the activity of a component to its mole fraction, given above, are specific examples of a more general way of relating activity to mole fraction. The activity and the mole fraction of a component in a solution are related through a parameter known as the activity coefficient. 

An ideal solution is an exception rather than the rule. Real solutions are, in general, nonideal. Any solution in which the activity of a component is not equal to its mole fraction is called non-ideal. The extent of the nonideality of a solution, i.e., the extent of its deviation from... 

The yeast-mediated enzymatic biodegradation of azo dyes can be accomplished either by reductive reactions or by oxidative reactions. In general, reductive reactions led to cleavage of azo dyes into aromatic amines, which are further mineralized by yeasts. Enzymes putatively involved in this process are NADH-dependent reductases [24] and an azoreductase [16], which is dependent on the extracellular activity of a component of the plasma membrane redox system, identified as a ferric reductase [19]. Recently, significant increase in the activities of NADH-dependent reductase and azoreductase was observed in the cells of Trichosporon beigelii obtained at the end of the decolorization process [25]. 

Equilibrium constants do not have units because in the strict thermodynamic definition of the equilibrium constant, the activity of a component is used, not its concentration. The activity of a species in an ideal mixture is the ratio of its concentration or partial pressure to a standard concentration (1 M) or pressure (1 atm). Because activity is a ratio, it is unitless and the equilibrium constant involving activities is also unitless. 

Furthermore, if the liquid is assumed to be ideal the activity of a component is equal to the mole fraction of the component. Now the mole fraction of i in the liquid phase can be derived as a function of the radius of the solid phase ... 

Dense membranes are used for pervaporation, as for reverse osmosis, and the process can be described by a solution-diffusion model. That is, in an ideal case there is equilibrium at the membrane interfaces and diffusional transport of components through the bulk of the membrane. The activity of a component on the feed side of the membrane is proportional to the composition of that component in the feed solution. 

So far, we have expressed the number of ions per unit volume by concentration. Physical chemists prefer to use activity to characterize the behavior of solutes in solution. As a solution becomes more concentrated and as the ionic strength increases, ions behave as if there were fewer of them present than would be indicated by their analytical concentrations. The activity of a component is related to its concentration by a proportionality constant known as an activity coefficient . Considerations regarding ionic activity become particularly important, when fluids are quite concentrated. An extension of this treatment is the use of the activity product to... 

The equilibrium constant defined by eqn. (26) can be used to calculate the equilibrium conversion of reactants to products under specified conditions of temperature and pressure. The activity of a component X in a mixture of ideal gases, Ox, is given by... 

The amount of water in the reaction mixture can be quantified in different ways. The most common way is to nse the water concentration (in mol/1 or % by volume). However, the water concentration does not give much information on the key parameter enzyme hydration. In order to have a parameter which is better correlated with enzyme hydration, researchers have started to nse the water activity to quantify the amount of water in non-conventional reaction media (Hailing, 1984 Bell et al, 1995). For a detailed description of the term activity (thermodynamic activity), please look in a textbook in physical chemistiy. Activities are often very nselul when studying chemical equilibria and chemical reactions of all kinds, but since they are often difficult to measure they are not used as mnch as concentrations. Normally, the water activity is defined so that it is 1.0 in pure water and 0.0 in a completely dry system. Thus, dilute aqueous solutions have water activities close to 1 while non-conventional media are found in the whole range of water activities between 0 and 1. There is a good correlation between the water activity and enzyme hydration and thns enzyme activity. An advantage with the activity parameter is that the activity of a component is the same in all phases at eqnihbrium. The water activity is most conveniently measnred in the gas phase with a special sensor. The water activity in a liqnid phase can thns be measured in the gas phase above the liquid after equilibration. 

The activity of a component in a solution is essentially a relative quantity. From the definition of activity it follows that the numerical value of the activity of a particular component is dependent on the choice of the standard state. There is no fundamental reason for preferring one standard state over another. Convenience dictates the choice of the standard state. Up to now, we have chosen the pure state as the standard state. That is, a pure component in its stable state of existence at the specified temperature and latm pressure is chosen as the standard state. This particular choice is known as the Raoultian standard state. 

Thus an electrochemical measurement enables the determination of the activity of a component in the solution and hence the partial molar free energy of the component. If we measure the emf in a range of temperature, we can determine S g and H g from the relationships developed in the previous section ... 

Assuming an ideal solution in which the activity of a component is identical to its concentration and no kinetic coupling occurs between individual fluxes, Equation 5.8 becomes identical with the Nernst-Planck flux equation [18], which is given by ... 

Metal with low ionization energy that loses electrons readily to form cations. Activity (of a component of ideal mixture)... 

It is evident from the equations (14) that the activity of a constituent of a solution can be expressed only in terms of a ratio of two chemical potentials, viz., m a>nd m, B d so it is the practice to choose a reference state, or standard state, for each constituent in which the activity is arbitrarily taken as unity. It can be readily seen from the equations given above that in the standard state the chemical potential m is equal to the corresponding value of m - The activity of a component in any solution is thus invariably expressed as the ratio of its value to that in the arbitrary standard state. The actual standard state chosen differs, of course, according to which form of equation (14) is employed to define the activity. 

Standard conditions Unit activity of a[[ components except H, which is maintained at 10" M. Gases are at 1 atm pressure. 

With the above definition, the total activity of a component in the solution is the sum of the... 

Activity (of a component of an ideal mixture) A dimensionless quantity whose magnitude is equal to molar concentration in an ideal solution, equal to partial pressure (in atmospheres) in an ideal gas mixture, and defined as 1 for pure solids or liquids. 

Equation (3.133) for the activity of a component derived in this way is completely universal and may be used in any system. The calculation of the activity of a component in an actual system using this equation thus respects the characteristic features of silicate melts, e.g. the different energetic state of individual atoms of the same kind. Such cases may... 

The activity of a component in a solution is dehned as the product of its activity coefficient, y, and its mole fraction, X ... 

The term coagulation factor deficiency is frequently ambiguous. Previously, it implied a deficit in the functional activity of a component because the clotting time tests measured only the ability of the component to support normal coagulation. The reference for normal functionality is the response of dilutions of plasma from pooled... 

Most liquid solutions, also called liquid mixtures, are non-ideal. This follows from the fact that the components are in intimate contact with one another, and that the forces between the various species are usually not the same. As a result, the physical properties of the solution, for example, the vapor pressure of a given component, are usually not simply related to its concentration. This non-ideality leads to the concept of the activity of a solution component. As far as the analytical chemist is concerned, only concentration is ultimately of interest. Thus, if an analysis is based on the measurement of a physical property which in turn depends on the activity of a component, it is very important that the relationship between activity and concentration be understood for the system in question. 

In order to determine the activity of a component in solution, one must measure its vapor pressure. In the case of volatile liquids such as those discussed in most of this chapter, vapor pressure measurement is not a problem so that very accurate determination of activity is possible over the whole composition range for which a solution is formed. However, many solutes, for example, most solids, have negligible vapor pressures. Under these circumstances, one makes use of the Gibbs-Duhem relationship between the activities of the two-components in solution. Since the vapor pressure of the solvent can be measured, its activity can be determined, and then used to estimate the activity of the solute. 

The activity of a component /, given by is defined in terms of chemical potentials ... 

In order to make our main equation useful for dealing with experimental data, it is necessary to introduce variables other than the chemical potentials. According to Lewis, the activity of a component in a uniform phase may be introduced by... 

To use Eq. (16.2), the significance of the function g (r, p) must be accurately described then a has a precise meaning. Two ways of describing gfT, p) are in common use each leads to a different system of activities. In either system the activity of a component is still a measure of its chemical potential. 

Examples of the activity of a component A (equal to the ratio of the partial pressure Pa over the alloy and the pressure over the pure component A) as function of the mole fraction are shown in Figure 3.6. 

Calculations of chemical equilibrium, which will be the topic of the next section, are facilitated through the introduction of the activity, a property closely related to fugacity and chemical potential. The activity of a component i in mixture is defined as the ratio of its fugacity over the fugacity of the same component at its standard state ... 

The activity of pure liquids and solids, under some specified standard conditions of temperature and pressure, is considered to be unity. The activity of a component in mixture systems is expressed as ... 

The activity of a component i at a given temperature, pressure, and composition can be defined as the ratio of the fugacity of the solvent at these conditions to the solvent fugacity in the standard state that is, a state at the same temperature as fliat of the mixture and at specified conditions of pressure and composition ... 

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